This invention relates generally to an electro-optical measuring system, and more particularly to a system which employs a laser beam to scan the object whose dimension is being measured to provide highly precise measurements.
Traditional gauging devices, such as micrometers and height gauges, all involve physical contact with the object being measured. The gauging procedure requires a trained operator and is often difficult and time-consuming. The widely used optical comparator technique in which a silhouette of the part is projected on a magnified scale onto a reference screen also dictates a trained operator and is subject to measurement errors due to eye fatigue, poor judgment and other factors. The rate of gauging by an optical comparator is limited by the ability of the operator to see and evaluate the image on the screen with respect to a reference outline.
In order to overcome the limitations of gauging procedures which entail human operators, non-contacting electronic measuring systems have been devised, many of which employ an electro-optical device such as a vidicon tube or an image dissector tube. These tubes, which include a photo-sensitive element, are adapted to electrically scan a light image or silhouette of the object projected onto the sensitive element to produce a video output that represents the dimension of the object being scanned.
One advantage gained by the use of an electronic scanning system is that the dimensions are obtained at a point remote from the part being gauged without the need for physical contact therewith. Also, measurements may be made to ascertain dimensions that are not readily accessible by physical contact.
One elementary form of electro-optical measuring system makes use of a light source which generates a fan-shaped light beam that is collected by a lens and focused on a photo-electric detector, the amount of energy falling on the detector being measured by standard photometric techniques. When the object to be measured is a cylinder of uniform diameter, such as a precision-ground shaft whose diameter is to be determined, the shaft is inserted in the fan-shaped light beam between the light source and the lens whereby the shaft intercepts a portion of the beam to an extent depending on its diameter. The approximate diameter of the shaft can then be determined by measuring the resultant reduction in energy picked up by the detector.
A more flexible electro-optical measuring system of greater precision is one using a light source producing a collimated beam to illuminate a field in which the shaft whose diameter to be measured is placed, the beam being projected toward a low distortion lens that develops a silhouette of the image generated by the shaft. This image is focused on the face of an electronic scanning tube or a standard T-V camera. An electro-optical arrangement of this type is disclosed in U.S. Pat. Nos. 3,854,822 and 3,902,811.
As explained in these patents, the scanner examines the images generated by the collimated light source and the imaging lens, and from this examination, it determines the dimensions of the shaft or other object being measured. It is to be understood that the shaft diameter is merely given as a very simple example of a dimension to be determined and that, in practice, the system disclosed herein may be used to measure any part dimension.
Other scanning systems have been constructed with rotating mirrors, prisms and gratings operating in conjunction with optical lenses and/or mirrors to produce a reasonably linear scan ranging from a few percent to 0.1% non-linearity.